Infective popliteal artery aneurysm by Streptococcus equi: An unusual pathogen

We report the case of a 63-year-old man who presented with a 2-week complaint of lower extremity pain, swelling, and low-grade fever after an episode of septic arthritis in the ipsilateral knee. The investigation showed a rapidly expanding popliteal artery aneurysm (PAA). The rare clinical entity of an infective PAA was suspected and was confirmed by the cultures obtained at the right femoropopliteal bypass with an autologous vein graft and subtotal resection of the aneurysm sac. Streptococcus equi was identified as the primary pathogen, which, to the best of our knowledge, has not been previously described for an infective PAA.

Infective aneurysms have been described in virtually every arterial bed and can occur as primary or secondary infections, with a predisposition to a location in the abdominal aorta. 1 Infective aneurysms can be due to bacteremia, local injury with inoculation, spread from neighboring tissues, 2 or septic emboli, usually secondary to endocarditis. 3 To date, <50 cases of primary infective PAA have been reported in the literature, with none due to Streptococcus equi, a species of gram-positive, coccoid bacteria isolated from the upper respiratory tract of horses. The patient provided written informed consent for the report of his case details and imaging studies.

CASE REPORT
A previously healthy 63-year-old white man who worked daily with horses was admitted to our hospital with a 2-week history of progressive right leg swelling and pain associated with intermittent low-grade fever. Two weeks earlier, he had had an episode of septic arthritis from his right knee with growth of Streptococcus equi on cultures of synovial fluid and blood.
This was sensitive to all tested antibiotics, including amoxicillin, cefadroxil, cefotaxime, erythromycin, isoxazolyl penicillin, clindamycin, benzylpenicillin, and penicillin V. He had no history of trauma, and the two blood cultures taken at the current admission again revealed growth of Streptococcus equi. The screening tests for human immunodeficiency virus and hepatitis were negative. Because of the progressive swelling of the right leg,

DISCUSSION
Streptococcus equi, subspecies Zooepidemicus, is a species of gram-positive, coccoid bacteria isolated from abscesses in submaxillary glands and mucopurulent discharge of the upper respiratory tract of horses. It is an opportunistic pathogen for both humans and a broad range of species, including horses, dogs, and pigs. 4 To the best of our knowledge, the present case is the first case reported of a symptomatic infective PAA with a pathogen most commonly known to affect equids  such as Streptococcus equi as the infectious pathogen. The patient's anamnesis and background revealed his potential exposure to this unusual pathogen for human infections because he worked daily with horses. Moreover, the recent anamnesis of septic arthritis with the same pathogen should raise awareness for this rare colonization of the PAA. The directness of this course of the disease could not be ascertained completely because the patient had most likely been contaminated with the pathogen through the respiratory track via inhalation while working near horses. This could have led to bacteremia and subsequently seeded the knee joint and the PAA.
The deep vein thrombosis was certainly a contributor to the patient's leg swelling. Its cause was most likely multifactorial, with compression of the popliteal vein by the rapidly expanding infective PAA. In addition, the local inflammation and bacteremia could have played a role. Venous DUS is the most common imaging modality of choice in the diagnostic evaluation of a patient with unilateral lower swelling and pain. However, in cases of potential infectious seeding, fluorine-18 fluorodeoxyglucose positron emission tomography/computed tomography can be very useful, as it was for the present case. The uptake was not limited to a perivenous area but instead to the aneurysm sac. Moreover, 16s rDNA sequencing analysis of bacterial taxonomy can be particularly useful, especially for patients already receiving antibiotic therapy. In retrospect, a more expedited diagnostic workup after the initial intravenous antibiotic course would have allowed for an earlier diagnosis and repair of the infective PAA without the final expansion. Open surgical repair with autologous material and debridement remains the treatment of choice for infective PAAs when the anatomy is favorable. Endovascular treatments have been described and still have a role but mainly for high-risk patients. 5,6 Because no specific treatment guidelines are available, management is commonly guided by the general principles of vascular surgery. No consensus has been reached in the literature regarding the approach for open revascularization, with medial and posterior approaches used equally and each offering separate advantages and disadvantages. The decision should be individualized; however, with local infection, there is increased difficulty with the posterior approach owing to the increased risk of bleeding and nerve damage. For our patient, the first issue was controlled by excluding the aneurysm before resection; however, temporary nerve damage still occurred.

CONCLUSIONS
An infective PAA is a rare, but potentially devastating, condition. A high index of suspicion and expedited diagnostic workup is, therefore, necessary, especially in the case of rapid enlargement of a PAA with an anamnesis of recent infections. A careful anamnesis taking could reinforce the suspicion of zoonosis even if not previously described, as in the present case. Surgical reconstruction using an autologous vein graft seems to be a feasible solution. Prolonged targeted antibiotic therapy and close follow-up protocols are recommended.